The Paleolithic diet, Paleo diet, caveman diet, or stone-age diet is a modern fad diet consisting of foods thought by its proponents to mirror those eaten by humans during the Paleolithic era. The diet avoids processed food and typically includes vegetables, fruits, nuts, roots, and meat and excludes dairy products, grains, sugar, legumes, processed oils, salt, alcohol, and coffee. Historians can trace the ideas behind the diet to "primitive" diets advocated in 19th century. In the 1970s Walter L. Voegtlin popularized a meat-centric "Stone Age" diet; in the 21st century the best-selling books of Loren Cordain popularized the Paleo diet. As of 2019 the paleo-diet industry was worth approximately US$500 million.